To make a complicated answer easy: If you have sufficient exhaust in the duct (aka case), the addition fans will increase flow. If there isn't enough exhaust, then the pressure will act against the fans and cause less airflow.
There are also concerns of the first fan's cfm being less than the second fan*, causing the first fan in the series to have no effect or burn the motor out, since it'll be spinning from the power of the oncoming air instead of the motor.
Do keep in mind that the physics are very dependent on the setup, so result may vary. I'd suggest experimenting for shortish periods to see if you get temperature changes.
*Fan setup (arrows indicate flow direction): [<-Case][<-Fan1][<-Fan2]